1. Field of the Invention
This invention relates to a spark plug, from which removal of unburnt carbon from the insulator surface is facilitated, and a process for producing same.
2. Prior Art
A gasoline motor vehicle, assembled in a factory, is usually moved a short distance from the factory to a motor pool or to some other place (for loading into a truck for shipment) by running a gasoline engine thereof prior to being dispatched to a user. The running mode for moving such a vehicle from the factory to any place is versatile. It is customary to run such a vehicle a short distance, e.g. in the order of 500 meters, and at a low speed, such as 30 to 40 km per hour, at each running. While running at a low speed for a short distance, a spark plug mounted in an engine tends to smolder, resulting in lowered insulation resistance of the insulator between the spark plug electrodes. This leads to misfiring and, hence, to defective running. Such a smoldering phenomenon is occasionally experienced in cold weather, such as in winter. The smoldering at a spark plug occurs not only with a new car but also with a used car, particularly in winter.
Such a smoldering phenomenon is caused by adhesion of unburnt carbon, such as carbon black, which accrues from the combustion of gasoline in an engine, to the surface of a taper-nose portion 61 of insulator 6 of a spark plug 1, which portion 61, as seen in FIG. 1, is exposed to combustion gases in an engine. FIG. 1 shows a housing 2, an electrode (ground) 3, a washer 4, and a central electrode 5.
To overcome the previously-described drawback, a ground was specially designed to use a creeping discharge to remove unburnt carbon from a spark plug. However, complications occurred in the construction of the periphery of the taper-nose portion 61 of insulator 6 and of the ground, and difficulties were encountered in the manufacture of such a spark plug.